1. Field of the Invention
The present invention generally relates to a flash device for use in association with a photographic camera and, more particularly, to a flash device of a type capable of initiating the firing in response to a flash firing command generated from the photographic camera and interrupting the firing in response to a flash terminating command generated from the photographic camera, the flash device itself, and so on, what is called an "auto strobe".
2. Description of the Prior Art
The flash device has long been well known of a type which comprises a flash tube and a thyristor so connected in series with the flash tube that the flash firing can be controlled by switching the thyristor on and off. An electronic circuit used in such prior art flash device is reproduced in FIG. 11 of the accompanying drawings in schematic block circuit representation. As shown, the thyristor SCR2 is connected in series with the flash tube Xe. When a firing command is generated from the camera body (not shown), a trigger circuit Tr is activated in response to the firing command and, simultaneously therewith, the thyristor SCR2 is switched on, thereby causing the flash tube Xe to fire. When a flash terminating command is generated from the camera body, or a control circuit built in the flash device, immediately after the amount of light emitted has attained a predetermined value, (that is at a timing T shown in FIG. 12) a thyristor SCR3 is switched on in response thereto and the circuit between the anode and cathode of the thyristor SCR2 is reverse-biased by the charge stored in a commutating capacitor C to switch the thyristor SCR2 off thereby to interrupt the flash firing of the flash tube Xe.
In this prior art flash device, since an electric current flows through the flash tube Xe, the commutating capacitor C and the thyristor SCR3 even though the flash terminating command has been generated and the thyristor SCR2 has subsequently been switched off, the flash of light tends to be excessively emitted from the flash device by the amount corresponding to the area hatched in the graph of FIG. 12.
Also, while the commutating capacitor C is charged through resistors r1 and r2, turn-off of the thyristor SCR3 requires the electric current flowing through the thyristor SCR3 to be of a value lower than the holding current and, therefore, the resistor r1 must have a relatively high resistance. This in turn brings about the increased time constant for the charging of the capacitor C and, accordingly, the repeated flash firing at short intervals cannot be accomplished.
Another prior art flash device of a type wherein a field effect transistor is so connected in series with the flash discharge tube that the field effect transistor can be switched on and off by controlling the voltage to be applied between the gate and source of the field effect transistor thereby to control the flash firing is disclosed in the Japanese Laid-open Patent Publication No. 61-50126 published in 1986. In this flash device of the above described type wherein the field effect transistor, or a similar transistor, is employed in place of the thyristor, the flash device itself tends to become bulky by the following reasons although the problems inherent in the prior art flash device shown in and described with reference to FIGS. 11 and 12 could have been eliminated.
(i) Where the transistor is employed, the amplification factor at both a high breakdown voltage and a high current region is so low that, in order for a high discharge current (about one hundred and several tens amperes) to pass from the flash tube, several amperes are required as a current for the control, resulting in the increased size of a circuit for the control of the transistor.
(ii) Where the field effect transistor is employed, the field effect transistor itself tends to become large in size at the high breakdown voltage and the high current region although the control circuit suffices to be small in size.
And further, recently, automation and diversification of photographic techniques have been advanced by allowing a photographic camera body with a microcomputer built therein and a flash device to communicate with each other. As a means for displaying information so communicated between the camera body and the flash device, the flash device is too provided with a data display unit which generally makes use of a liquid crystal display for the purpose of minimizing the electric power consumption. However, the liquid crystal display becomes invisible when in the dark environment and, therefore, the use of a back-lighting device is necessitated to illuminate the liquid crystal display.
As disclosed in, for example, the Japanese Laid-open Utility Model Publication No. 62-116227, an electroluminescent device is generally employed as a light source for the back-lighting the liquid crystal display. Since the electroluminescent device is in the form of a thin film, the use thereof makes it possible to manufacture the display unit compact. Moreover, the electroluminescent device may be referred to as a planar light source, i.e., a light source capable of emitting light from a substantially entire surface area thereof, in contrast to a bulb which may be regarded as a pin-point light source, and, therefore, no substantial fading take place in the display unit. As is well known, the electroluminescent device emits light as a result of repetition of charging and discharging action of a capacitance component thereof.
It has, however, been found that the additional incorporation of a control circuit for controlling the electroluminescent device, that is, for effecting the charging and discharge of the capacitance component of the electroluminescent device, tends to bring about an increased cost and also an increased size of the flash device as a whole, with the consequence that not only is the portability of the flash device lowered, but the handling convenience thereof is also lowered.